Drug absorption is typically predicted based on tests designed to evaluate oral administration. There are several distinct steps in modeling or understanding drug absorption including release from the excipient; solvation in the body fluid; absorption by the luminal or mucosal cells and distribution to local or systemic sites. (Shono Y, Jantratid E, Kesisoglou F, Reppas C, Dressman J B. “Forecasting in vivo oral absorption and food effect of micronized and nanosized aprepitant formulations in humans.” Eur J Pharm Biopharm. pp 95-104 September; 76(1), (2010))
The first step is the release of the drug from the excipient or carrier. This is universally tested by dissolution. The drug and excipient combination are chosen to dissolve in the target body fluid. For instance, a sublingual delivery would be optimized to dissolve in saliva, while a stomach delivery would be designed to be released in the acidic milieu of the gastric juices. Enteric coatings that are designed to resist dissolution in an acidic environment are used to deliver drugs to the small intestine. Suppositories are designed to dissolve in the rectal fluid (or to melt), for example from hydrophobic carriers such as cocoa butter, fatty acids or hydrogenated vegetable oils
Oral delivery systems include tablets, gel capsules and capsules. These can contain excipients such as binders (e.g., hydroxypropylmethylcellulose, polyvinyl pyrilodone, other cellulosic materials and starch), diluents (e.g., lactose and other sugars, starch, dicalcium phosphate and cellulosic materials), disintegrating agents (e.g., starch polymers and cellulosic materials) and lubricating agents (e.g., stearates and talc).
Transmucosal delivery systems include patches, tablets, suppositories, pessaries, gels and creams, and can contain excipients such as solubilizers and enhancers (e.g. propylene glycol, bile salts and amino acids), and other vehicles (e.g., polyethylene glycol, fatty acid esters and derivatives, and hydrophilic polymers such as hydroxypropylmethylcellulose and hyaluronic acid).
Dermal delivery systems include, for example, aqueous and nonaqueous gels, creams, multiple emulsions, microemulsions, liposomes, ointments, aqueous and nonaqueous solutions, lotions, aerosols, hydrocarbon bases and powders, and can contain excipients such as solubilizers, permeation enhancers (e.g., fatty acids, fatty acid esters, fatty alcohols and amino acids), and hydrophilic polymers (e.g., polycarbophil and polyvinylpyrolidone). In one embodiment, the pharmaceutically acceptable carrier is a liposome or a transdermal enhancer.